Lingerie has come a long way since the 1920s, when the shorter skirts preferred by the flappers required underwear to get a major makeover: Before mini hemlines were en vogue, split-crotch, “open gusset” knickers were the airy norm. They were considered appropriate for hygiene, but not for high kicks in jazzy speakeasies, thus ushering in the closed-crotch style still favored today.

Author Cora Harrington, who has been dishing about unmentionables for over a decade on her popular blog the Lingerie Addict, includes such fascinating historical tidbits in her new book, “In Intimate Detail: How to Choose, Wear, and Love Lingerie.” The guide to all-things-underwear expands on her mission to help women of all shapes and sizes feel comfortable around pretty, clingy, lacy things.

“Lingerie is just so complicated, it’s endlessly fascinating,” the 34-year-old Harlem resident tells The Post. To prove it, she takes readers on an illustrated journey from Victorian “bum rolls” — padded pillow-belts that ladies wore to accentuate their hips and make their waists look slimmer — to contemporary contoured (and comfy) T-shirt bras.

To Harrington, lingerie is about more than sex appeal. It reflects our deepest desires about our bodies: “In every era, lingerie changes to support or even create the fashionable figure of that day,” she writes.

Here, the Lingerie Addict models some of the most noteworthy underwear styles of the last century — that you can still shop today — and describes how they’ve adapted to reflect our ever-changing physical ideals.

1920s

Brian Zak/NY Post

The trend: Tap pants
After World War I, Jazz Age girls stopped striving for hourglass figures and embraced a straighter, more boyish look. The flappers caroused in box-cut, knee-length dresses that allowed for plenty of movement. Their underwear was equally breezy (though closed-crotched); unlike the corsets of previous eras, tap pants are loose and light with no internal structure. “That was a really big shift,” Harrington says. “For hundreds of years, having these heavily boned, heavily structured undergarments for women was just the norm.”

1950s

Brian Zak/NY Post

The trend: Corselets
At the height of World War II, women couldn’t buy things like corsets or stockings because the materials used to make them — steel and nylon — were diverted to the Army. But after the war, gorgeous lingerie became available again. More voluptuous bodies, a la the pinups, were celebrated. The corselet, which is a tight-fitting, heavy-duty girdle dress, gives a woman curves reminiscent of the sultry Bettie Page. “A lot of people would have worn this every day,” Harrington says, “because it gives you that classic pinup shape.”

1960s

Brian Zak/NY Post

The trend: Pantyhose
The widespread embrace of miniskirts meant that women’s garter belts — used to hold up slippery thigh-high stockings — might stick out. Pantyhose, which come up over the hips, were created to solve this problem. The youthful mod era also spelled the end of the girdle, which became associated with old-fashioned notions of beauty. “In the ’60s, there was a shift in how people thought about undergarments,” says Harrington. “The emphasis changed from using shapewear to create a taut, toned figure, to achieving this through diet and exercise.”

1980s

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The trend: Bodysuits
In the Reagan era, mile-long legs and smooth hips were the coveted combination du jour, embodied by statuesque supermodels such as Elle Macpherson and Carol Alt. High-cut bodysuits, as well as leotards, highlighted both. Popular fitness regimens such as Jane Fonda’s Workout and Jazzercise helped women tone their bodies with unprecedented zeal. “The ’80s were all about excess,” says Harrington, citing the slinky looks that were big at the time. “We can’t just have a leg opening, it has to be an extreme leg opening.”

2018

Brian Zak/NY Post

The trend: Unlined bras
After the push-up bra “heyday” of the ’90s and early aughts, Harrington believes that women are now willing to test out more natural-looking styles, such as bralettes for petite women, and more supportive, seamed bras for bustier ladies. This, she says, is a direct result of the cultural movement around body positivity and self-acceptance, where women want to feel confident in their own skin. “I think people are interested in a look that accentuates and frames their natural bust, as opposed to adding or taking away,” Harrington says.